This application is based on applications Nos. 2001-197029 and 2001-331280 filed in Japan, the contents of which are incorporated hereinto by reference.
The present invention relates to an etching method for wet etching a laminated assembly having a polyimide layer.
Multilayer structures have found a great variety of applications, the structure comprising laminations of an insulative resin film and a metal layer imparting rigidity and defining a conductive circuit. Of the resin materials for forming the film, polyimide resins are widely used because of their superior heat resistance, dimensional stability, resistance to agents, electrical properties and mechanical properties. For instance, the polyimide resin is used in flexible printed circuits, film carriers employed for TAB (Tape Automated Bonding) or COF (Chip on film) mounting, and flexures with wiring for use in magnetic head suspensions and the like.
A method for fabricating the laminated assembly comprising the non-thermoplastic polyimide film and the metal layer includes (a) one wherein a metallic circuit layer is formed on the aforesaid resin film by spattering, electrolytic plating or the like; (b) one wherein a liquid-like non-thermoplastic polyimide (or polyamic acid as the precursor thereof) is applied to the metal layer by coating or casting; and (c) one wherein laminations of the resin film and the metal foil bonded together via an adhesive layer is processed.
The above methods each have an advantage and disadvantage and are selectively adopted according to applications. The method (a) has advantages of enjoying a great number of applicable non-thermoplastic polyimides and accomplishing an excellent fine patterning characteristic in circuit formation but suffers a disadvantage of low adhesion between the non-thermoplastic polyimide layer and the metal layer. The method (b) assures a good adhesion between the non-thermoplastic polyimide layer and the metal layer but limits the applicable non-thermoplastic polyimide to those having low thermal expansibilities (expansibilities close to that of the metal). On the other hand, the method (c) has advantages of good adhesion between the non-thermoplastic polyimide layer and the metal layer and a relatively large number of applicable non-thermoplastic polyimides. However, the method uses an adhesive material having lower heat resistance and electrical properties than the non-thermoplastic polyimide, so that the resultant products suffer poor characteristics as well as low resistance to agents and heat.
Conventionally, an epoxy-based or acrylic adhesive is widely used for forming the adhesive layer which is essential in the method (c). More recently, however, the use of an adhesive layer of thermoplastic polyimide is increasing as an approach to enhance the above advantages and to improve the poor characteristics attributable to the adhesive layer.
In the laminated assembly comprising the non-thermoplastic polyimide layer and metal layer, the formation of hole in the polyimide layer is generally done by laser processing, plasma processing (dry etching), wet etching or the like. Among these, the plasma processing provides a smoothly etched surface, as shown in FIG. 4. However, this method requires a special equipment and hence, is not so suitable for general purpose use in terms of economy. Because of the isotropic nature, the wet etching process has a drawback of etching away a portion under the metal mask layer (see FIG. 1). However, this method is widely used because the polyimide layer can be etched quickly and economically. Hence, there have been proposed a great variety of etching solutions comprising an alkali metal hydroxide, alcohol, phenol, amine compound, amide compound, oxyalkylamine, hydrazine or the like (see, for example, Japanese Unexamined Patent Publication No. 10(1998)-97081).
Unfortunately, where the conventional etching solution is applied to the laminated assembly including the non-thermoplastic polyimide layer and the metal layer bonded together via the adhesive layer of thermoplastic polyimide, a favorably etched feature cannot be obtained because the thermoplastic polyimide is etched at much lower etching rate than the non-thermoplastic polyimide so that great pits and projections are produced on a side of an etched portion, as shown in FIG. 3. It is noted that the figure shows, in vertical section, one typical side of an etched laminated assembly, wherein P1 indicates the non-thermoplastic polyimide layer; P2 indicates the adhesive layer of thermoplastic polyimide; and K indicates the metal layer.
As to the etching of the polyimide layer, an attempt has been made to simplify the complicated etching process by employing a dry-film resist layer as a resist mask instead of a metal mask. The metal mask means a resist mask for polyimide etching, which is produced by etching the metal layer or by depositing a metal layer on the laminated assembly by sputtering or the like to form a mask thereon. Where the metal mask is produced by etching the metal layer, the polyimide etching process includes the steps of producing the metal mask and then etching the polyimide layer, the metal mask produced by exposing a resist to light, developing the resist, etching a metal layer and removing the resist.
If, on the other hand, the dry film resist can be used in the polyimide etching process, the process is simplified because the steps of exposing and developing the resist are followed by etching the polyimide layer. Unfortunately, if the dry-film resist is formed according to the conventional conditions, there is a drawback that the adhesion between the dry film and the polyimide layer is decreased during the etching process, resulting in the separation of the dry film from the polyimide layer.
In view of the foregoing, it is an object of the present invention to provide a wet etching method which accomplishes a smoothly etched feature of the laminated assembly comprising the non-thermoplastic polyimide layer and the metal layer bonded together via the adhesive of thermoplastic polyimide without producing difference between an etching amount of the thermoplastic polyimide portion and that of the non-thermoplastic polyimide portion, and which assures an etching process free from inter-layer delamination despite the application of the dry-film resist layer to the process.
The present inventors have made intensive study to achieve the above object, focusing on the compositions of the etching solution to find that the following etchant permits the etching process to proceed without producing difference between an etching amount of the thermoplastic polyimide portion and that of the non-thermoplastic polyimide portion. That is, the etchant contains an alkali metal hydroxide, oxyalkylamine and water and is prepared in a particular manner that the concentrations of the alkali metal hydroxide and water satisfy specific relationships. The inventors have made further investigation based on the finding thereby to achieve the present invention.
According to the invention, a method for etching a laminated assembly is characterized in that a laminated assembly comprising a metal layer and a non-thermoplastic polyimide layer bonded together via thermoplastic polyimide is etched using an etchant at least containing an alkali metal hydroxide, water and oxyalkylamine,
the concentrations of the alkali metal hydroxide (X weight %) and of the water (Y weight %) having relationships represented by coordinate points present within a region (inclusive of boundary lines) defined by the following expressions [1] and [2]:
Y=(xc2xd)X (provided that 7xe2x89xa6Xxe2x89xa645)xe2x80x83xe2x80x83[1]
Y=({fraction (5/20)})X+17.5 (provided that 7xe2x89xa6Xxe2x89xa645)xe2x80x83xe2x80x83[2]
provided that X and Y are defined based on the total weight of the alkali metal hydroxide, water and oxyalkylamine expressed as 100.
FIG. 5 is a graph representing the above relationships of the concentrations of the components. In the figure, xe2x97xaf represents each of Examples 1-9 plotted between the concentrations of potassium hydroxide and water, whereas X represents each of Comparative Examples 1-4 plotted the same way. That is, the concentrations of the alkali metal hydroxide and water in the etchant according to the invention are represented by coordinate points present within a crosshatched region (inclusive of boundary lines) in the chart. Besides the above two components, the etchant contains oxyalkylamine as a principal component. However, the etchant may further contain a suitable small amount of additive as required. The use of the etchant featuring the concentrations of the components so related permits the etching of the laminated assembly to proceed without difference between the etching rates at these polyimide layers, thus providing a smoothly etched feature in short time.
If the concentrations of water are under the line of the expression [1], the water is insufficient in amount for dissolving the alkali metal hydroxide and oxyalkylamine. Furthermore, such an etchant presents a low etching rate and hence, is not preferred. Conversely if the concentrations of water are above the line of the expression [2], such an etchant provides an excessive etching residue and hence, is not preferred. On the other hand, if the alkali metal hydroxide is present in concentrations of less than 7 weight %, such an etchant presents a low etching rate and hence, is not preferable. If the concentrations of the alkali metal hydroxide exceed 45 weight %, the region defined by the lines of the expressions [1] and [2] is so narrow that the preparation of the etchant is difficult. In addition, such an etchant is susceptible to variations of the compositions thereof while used (because of the evaporation of the liquid during the etching process). Therefore, the etchant is not preferred.
According to the present invention, potassium hydroxide may preferably be used as the alkali metal hydroxide whereas ethanolamine may preferably be used as the oxyalkylamine.
Preferred as the non-thermoplastic polyimide is polypyromellitimide polyimide. Particularly preferred as the metal layer are a copper thin film and stainless steel layer.
Where the inventive method is applied to a process where the polyimide layer of the laminated assembly is covered with a dry-film resist layer as an etching mask, there occurs no inter-layer delamination. Therefore, the inventive method provides a remarkable contribution to the reduction of steps in contrast to a process where the metal mask is used.
It is preferred that the dry-film resist layer is exposed to ultraviolet rays in an amount of 10 to 13 steps as measured with Stouffer 21-step tablets available from Stouffer Graphic Arts Equipment Co., and then developed. This ensures that the adhesion between the resist layer and the polyimide layer is increased.
It is preferred that the dry-film resist layer is further subjected to post-baking and/or re-exposure to light. This is also effective to increase the adhesion between the resist layer and the polyimide layer. It is preferred that the resist layer is re-exposed to twice as much ultraviolet rays as in the previous light exposure step.